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Home > Archive > Electrical Engineering > July 2005 > AC Motor braking resistors
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| Author |
AC Motor braking resistors
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| Hello, can someone give me a explanation on how braking resistors work on AC
motors controlled by a VFD?
Thanks, Dave
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| me@home.com 2005-07-05, 11:25 pm |
| The only place for motor energy to go is into the load or into the
resistor. To slow the motor the drive the energy has to go somwhere,
its only path is back through the IGBT or similar device back to the
DC bus, where a resistor bleeds off the excess energy to keep the DC
voltage under control.
The AC/DC converter on the front end is different for a regenerative
AC drive. SCRs vs. diodes. It lets you shut off or otherwise control
the incoming power to control the braking.
On Tue, 05 Jul 2005 23:58:49 GMT, "Dave" <davco1@telus.net> wrote:
quote:
>Hello, can someone give me a explanation on how braking resistors work on AC
>motors controlled by a VFD?
>
>Thanks, Dave
>
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| Correct me if I'm wrong but I believe that AC motors are braked by
introducing a DC current across the stator windings??
Fred
<me@home.com> wrote in message
news:dhbmc1hp68hq14o2a7m7gsvqlet9lmt358@4ax.com...
quote:
> The only place for motor energy to go is into the load or into the
> resistor. To slow the motor the drive the energy has to go somwhere,
> its only path is back through the IGBT or similar device back to the
> DC bus, where a resistor bleeds off the excess energy to keep the DC
> voltage under control.
>
> The AC/DC converter on the front end is different for a regenerative
> AC drive. SCRs vs. diodes. It lets you shut off or otherwise control
> the incoming power to control the braking.
>
> On Tue, 05 Jul 2005 23:58:49 GMT, "Dave" <davco1@telus.net> wrote:
>
>
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| Ted Rubberford 2005-07-06, 6:25 pm |
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"Fred" <KirkJohnson@shaw.ca> wrote in message
news:syNye.1875618$6l.883601@pd7tw2no...
quote:
> Correct me if I'm wrong but I believe that AC motors are braked by
> introducing a DC current across the stator windings??
>
> Fred
Read the post again boy, "motors controlled by a VFD"
Teddy Rubberford.
Bizarre Bugger...
quote:
>
> <me@home.com> wrote in message
> news:dhbmc1hp68hq14o2a7m7gsvqlet9lmt358@4ax.com...
>
>
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| Actually, most VFDs do both. Dynamic Braking, i.e. using the braking
resistors, is a good way to bring the motor down CLOSE to a stop, but
as the motor slows, the energy in it becomes less andless, so the
braking energy that can go into the resistors becomes less as well. So
at some point, the law of diminishing return takes over, and you have
to finish the job using DC injection braking.
On Wed, 6 Jul 2005 18:47:57 +0100, "Ted Rubberford"
<rubberford@hotmail.com> wrote:
quote:
>
>"Fred" <KirkJohnson@shaw.ca> wrote in message
>news:syNye.1875618$6l.883601@pd7tw2no...
>
>Read the post again boy, "motors controlled by a VFD"
>
>Teddy Rubberford.
>Bizarre Bugger...
>
>
>
>
>
>
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| Ignacio Simón Yarza 2005-07-17, 12:25 pm |
| Hello,
I will pipoint some issues that may clarify the facts:
1) VFD FOC techniques try to get equivalent behaviour to inverse braking but
in such a way that differential speed between stator and rotor magnetic
fields is controlled. A lagged field which makes rotor speed slow down is
produced on stator windings. But then, Where does the kinetic energy goes?
2) These rotor and stator magnetic fields result in two differential
voltages. The difference between them yields a voltage vector which
generates a current flowing back into the VFD. This is an energy returning
path which accounts for the dissipation of kinetic energy on the rotor axle.
3) The additional point to be clarified is why using this braking technology
instead of using a magnetic field acting proportionally to axle speed?? just
for the same reason for which VFD are used instead of direct feeding of
motor. As FOC theories allow treating AC motors as if they were DC
control-prone motors, so does VFD braking. In case Thyristor bridge is well
dimensioned, any braking curve may be obtained. On the contrary, DC braking
is cheaper but on its basic development it only allows a damped braking,
with a counter-force proportional to speed, and a mechanical brake works
always at the end of the braking cycle.
Best Regards.
Ignacio Simón Yarza.
Mech eng
Electronic and automatism eng
"Bob" <licku45@yqahoo.com> escribió en el mensaje
news:od84d1lfflftmkebostclpm3v7q5fguqds@4ax.com...
quote:
> Actually, most VFDs do both. Dynamic Braking, i.e. using the braking
> resistors, is a good way to bring the motor down CLOSE to a stop, but
> as the motor slows, the energy in it becomes less andless, so the
> braking energy that can go into the resistors becomes less as well. So
> at some point, the law of diminishing return takes over, and you have
> to finish the job using DC injection braking.
>
> On Wed, 6 Jul 2005 18:47:57 +0100, "Ted Rubberford"
> <rubberford@hotmail.com> wrote:
>
>
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